ANTAKYA, Turkey - Syria's conflict spilled over into Turkey on Monday and government forces battled rebel fighters near the border in clashes that appeared to doom a ceasefire less than 24 hours before a UN-brokered deadline for a Syrian army pull-back.

Under a deal brokered by UN-Arab League envoy Kofi Annan, Syria is to begin withdrawing troops from around urban centers by Tuesday, paving the way for a truce to start 48 hours later.

But the prospect of a ceasefire looked increasingly dim, with no let-up in violence around the country where forces loyal to President Bashar al-Assad have battled to crush a popular revolt against his rule for more than a year.

In a last-minute move, Assad has demanded written guarantees from opposition fighters that they put down their weapons, prompting Turkey's Deputy Foreign Minister Naci Koru to say that the April 10 deadline was now effectively void.

"April 10 has become void. After Kofi Annan's visit tomorrow (to Turkey) a new stage will start," state-run broadcaster TRT quoted Koru as saying on its website.

European Union foreign policy chief Catherine Ashton said through a spokesman that "adding new conditions at this stage is totally unacceptable".

Less than 24 hours before the deadline, fierce fighting raged on the ground, with two Syrian refugees and a Turkish translator wounded by gunfire from Syria at a refugee camp on Turkish territory, according to Turkish officials.

It was not clear if the camp was deliberately targeted or hit by stray bullets.

In clashes inside Syria near the Turkish border, rebels killed at least six members of the Syrian security forces and customs officials, according to the opposition Syrian Observatory for Human Rights.

It said the fighting took place in the village of Salama, between the Syrian town of Azaz and the Turkish town of Kilis. Eight rebel gunmen were wounded in the fighting.

The state news agency SANA said nine law-enforcement members and a civilian were shot dead and 13 wounded in al-Sukkari region in Aleppo.

State forces had gone "to protect a protest gathering after several attacks on such gatherings" were wrongly blamed on army and police, SANA said. They were shot down from all sides.

The agency said security forces on Sunday foiled infiltration attempts of two armed groups from Lebanon.

In Damascus province, four soldiers were killed in the bombing of a convoy as Syrian forces swept villages arresting opposition suspects. Two policemen were killed in clashes with gunmen in the city of Aleppo, the British-based Observatory said.

Colonel Qassem Saad al-Deen, spokesman for the Free Syrian Army in Syria, said on Sunday that at least 1,000 people had been killed during the past week, most of them civilians.

Deadline

Western powers suspect Assad is using the time since he met Annan in Damascus a month ago to mount a military offensive aimed at clearing the country of rebel strongholds.

Since the outbreak of the uprising in March 2011, Syria has blamed the unrest on foreign-backed terrorists determined to use violence to destabilize the government. Assad, who has ruled for 10 years since succeeding his late father, Hafez al-Assad, has laid out his own reform program but it has been dismissed by the opposition.

The severity of Assad's crackdown, in which the United Nations says 9,000 people have been killed, has triggered Western condemnation and sanctions, as the Syrian economy ground to a halt and its pound halved in value.

Under the UN plan, the Syrian government and opposition must stop fighting at 6 a.m. (0300 GMT) on April 12, if Damascus meets its deadline 48 hours earlier to pull back troops from cities and cease using heavy weapons.

Russia and China - who have so far sided with Assad - urged both sides to stop fighting and give Annan's peace plan a chance to work.

Reports from anti-Assad activists say the army is trying to seize back swathes of northwestern Idlib province from rebel control, using tanks and helicopters and driving terrified villagers north and west to the Turkish frontier.

The Observatory on Sunday reported shooting and shelling in a number of cities. At least 21 people were killed in Homs, Deraa, Idlib, Deir al-Zor and Hama province, and at least 12 government soldiers died in clashes, it said.

Syria has placed tight restrictions on media access, making it hard to verify witness accounts.

Russia, Assad's most important ally, stopped short of pressing him to rein in his army.

"Attempts to force a solution on Syria from outside will lead only to an escalation of tension," Russian Deputy Foreign Minister Gennady Gatilov said. "Everything must follow from respect for Syria's sovereignty, and violence must be stopped."

Syria on Sunday put up an obstacle to implementation of the truce on Sunday by saying it wanted written guarantees from opposition forces.

"Syria has a plan for a military pullback already in place and being implemented, but completing and achieving the main goal would definitely require the guarantees from the other side and those supporting them to abide by the terms of calm," Foreign Ministry spokesman Jihad Makdissi said in a statement.

A rebel commander said this was just a smokescreen.

"The regime will not implement this plan. This plan will fail," the Free Syrian Army (FSA) chief Colonel Riad al-Asaad told Reuters. "We have given our word that if the regime commits to the plan then we will too."

"We will never hand over our weapons," he added. (Additional reporting by Tulay Karadeniz, Jonathon Burch in Turkey, Steve Gutterman in Moscow, Michael Martina in Beijing, Justyna Pawlak in Brussels)